1. Field of the Invention
The invention relates generally to electrical control systems, and more particularly to systems and methods for providing electrical power to downhole oil production equipment such as electrical submersible pumps.
2. Related Art
Electric submersible pumps are typically used to pump crude oil out of wells that may be thousands of feet deep. These pumps are driven by power systems that are at the surface of the wells and are connected to the pumps by thousands of feet of electrical cable. The power systems may, for example, include variable speed drives that can control the speed of the pump motors and thereby control the speed at which the oil is pumped out of the wells.
A variable speed drive normally receives low-to-medium-voltage AC power, converts the AC power to DC which charges a capacitor bank, and then draws energy from the capacitor bank to produce the desired high-voltage AC output power that is supplied to the pump motor. Step-up transformers were initially used to convert relatively low inverter output voltages to the higher desired voltages but, more recently, systems have been developed which utilize power cells or modules that are connected in series to directly generate the high-voltage output.
The output of a variable speed drive for an electric submersible pump is typically a PWM (pulse width modulated) signal. A PWM signal is a square wave that has pulses of varying widths. The widths of the pulses are modulated to achieve a desired duty cycle for each pulse. By varying the duty cycle over time, the output voltage of the PWM signal can approximate a desired waveform, such as a sinusoidal waveform. In systems which use serially connected power cells to generate high output voltages, the output voltages of the power cells may be added to create a stepped waveform, rather than a simple binary (on-off) waveform.
Systems that have serially connected power cells may be designed so that the switching of each of the serially connected power cells is staggered in time with respect to the other power cells to which it is serially connected (see, e.g., U.S. Pat. No. 5,625,545 to Hammond.) By staggering the switching times of the cells, the harmonic components of the output voltage can be reduced. Also, because the maximum output voltage is reached in several steps, rather than a single step, the rate of rise (dv/dt) of the voltage signal is less than if the maximum voltage where reached in a single step, so less noise is generated and there is less overshoot of the maximum voltage at the pump motor.
There may, however, be circumstances in which it is desirable to use serially connected power cells that have switching times which are not staggered. In a system such as this, the maximum output voltage is reached in a single step. The rate of rise of the output voltage signal in this case is much larger than if the maximum voltage where reached in multiple steps, causing more noise and more overshoot of the maximum voltage at the pump motor. Because the noise and overshoot are undesirable and potentially harmful to the pump motor, it would be desirable to provide systems and methods for reducing these effects of switching in the power cells.